Pneumatic tyre

Abstract: A pneumatic tyre comprising a tread portion (5), a pair of axially spaced bead portions (3) with a bead core (2) therein, a pair of sidewall portions (4), a carcass (6) extending between the bead portions (3), a belt (7) disposed radially outside the carcass (6) and inside the tread portion (5), characterised in that in a meridian section of the tyre when mounted on a standard rim but not inflated, the carcass line (L) defined by the thickness centre line of the carcass (6) comprise a tread crown part (Q1) beneath the belt (7), a pair of upper sidewall parts (15) radially outward of the maximum carcass width position, and a pair of shoulder parts (Q3) between the tread crown part (Q1) and the upper sidewall parts (15), the tread crown part (Q1) having a first single radius of curvature R1, the upper sidewall parts (15) having a second single radius of curvature R2, and the shoulder parts (Q3) having a third single radius of curvature R3, the third radius of curvature R3 being in the range of from 0.003 to 0.03 times the first radius of curvature R1, the third radius of curvature R3 being in the range of from 0.05 to 0.5 times the second radius of curvature R2, the axial position of the boundary (Po) between the tread crown part (Q1) and each shoulder part (Q3) being between the axial position of each edge (7e) of the belt (7) and a position which is 20 mm axially inward thereof.

Description: The present invention relates to a pneumatic tyre having an improved carcass line which is capable of reducing the transmission of vibration during running and thereby reducing road noise. Tyres are vibrated during running by roughness of the road surface, and the vibrations are transmitted from the tyre tread portion to the car body through the tyre sidewall and bead portions and the wheel to generate so called road noise whose peak lies around 200-250Hz. In a radial tyre provided with a stiff belt, e.g. a steel belt, road noise is remarkably large. In order to reduce the road noise, the tread rigidity has been decreased by decreasing the tread rubber hardness and/or belt cord modulus. Further, it has been proposed to provide inside a tyre a vibration damper layer made of a foamed rubber. However, when the tread rigidity is lowered, the cornering force is inevitably decreased which deteriorates steering performance. On the other hand, when a damper layer is disposed in the tyre, the dynamic balance of the tyre is liable to be disturbed, and the ride comfort is deteriorated. It is therefore an object of the present invention to provide a pneumatic tyre in which, road noise can be reduced without lowering the tread rigidity and without using an additional damper material. According to the present invention, a pneumatic tyre comprises a tread portion, a pair of axially spaced bead portions with a bead core therein, a pair of sidewall portions, a carcass extending between the bead portions, and a belt disposed radially outside the carcass and inside the tread portion, wherein in a meridian section of the tyre when mounted on a standard rim but not inflated, the carcass line defined by the thickness centre line of the carcass comprises a tread crown part beneath the belt, a pair of upper sidewall parts radially outward of the maximum carcass width position, and a pair of shoulder parts between the tread crown part and the upper sidewall parts, the tread crown part having a first single radius of curvature R1, the upper sidewall parts having a second single radius of curvature R2, and the shoulder parts having a third single radius of curvature R3, the third radius of curvature R3 being in the range of from 0.003 to 0.03 times the first radius of curvature R1, the third radius of curvature R3 being in the range of from 0.05 to 0.5 times the second radius of curvature R2, the axial position of the boundary between the tread crown part and each shoulder part being between the axial position of each edge of the belt and a position which is 20 mm axially inward thereof. By setting the first, second and third radii R1, R2 and R3 as explained above, the carcass line has a more squared profile rather than a round profile in the non-inflated state. As the inner pressure of the tyre is increased to a normal pressure, the carcass line is changed such that the tread crown part and upper sidewall parts swell or move outwardly of the tyre. However, as shown in Fig.3, the shoulder parts between the crown part and sidewall parts move inwardly of the tyre, and the tension of the carcass cords in the inflated condition is partially decreased in the shoulder parts. As a result, the vibration transmissibility from the tread portion to the bead portions especially in a frequency range of under 200-250 Hz is decreased, and road noise is effectively reduced. Embodiments of the present invention will now be described in detail, by way of example only, in conjunction with the accompanying drawings. Fig.1 is a cross sectional view of an embodiment of the present invention; Fig.2 is a diagram showing the carcass line thereof; Fig.3 is a diagram showing the change in the carcass line of the present invention from zero inner pressure to normal pressure; Fig.4 is a schematic cross sectional view of another embodiment of the present invention; Fig.5 is a diagram showing the change in the carcass line of a prior art tyre from zero inner pressure to normal pressure; and Fig.6 is a graph showing the vibration transmissibility of a tyre of the present invention as a function of frequency. In Fig.1, a pneumatic tyre 1 comprises a pair of axially spaced bead portions 3 with a bead core 2 disposed therein, a sidewall portion 4 extending radially outwardly from each of the bead portions 3, a tread portion 5 extending between the radially outer ends of the sidewall portions 4, a carcass 6 extending between the bead portions 3 through the tread portion 5 and sidewall portions 4 and turned up around the bead cores 2 from the inside to outside of the tyre, and a stiff belt 7 disposed radially outside the crown portion of the carcass 6 and inside the tread portion 5. In this embodiment, the pneumatic tyre 1 is a passenger radial tyre having an aspect ratio (tyre height/tyre width) of not more than 0.70. The tyre mounted on a regular rim F but not inflated has a profile which is substantially same as that in the vulcanising mould. The tyre is mounted on the rim F by filling the tyre with pressurised air as usual, and then the air is released to zero pressure. The carcass 6 comprises at least one ply, in this example only one ply 6a, of cords arranged at an angle of 75 to 90 degrees with respect to the tyre equator C. For the carcass cords, organic fibre cords, e.g. polyester, nylon or the like are preferably used. In this example, polyester fibre cords are used. As the carcass 6 is turned up, a toroidal main portion 6A extending from one bead core to the other bead core and a pair of turned up portions 6B are formed. The height of the turned up portions 6B is larger than the height HF of the flange of the rim F. Between the toroidal main portion 6A and each turned up portions 6B, a bead apex 8 made of hard rubber tapering radially outwardly from the bead cores 2 is disposed. The height HB of the radially outer end of the bead apex 8 from the bead base line BL is smaller than the height HM of the maximum width position M from the bead base line BL at which position the maximum width of the carcass 6 lies. The radially outer end of the turned up portion 6B is terminated on the axially outer face of the bead apex 8 so that the carcass has a so called low turnup structure. In this embodiment, the belt 7 consists of a radially inner ply 7a and a radially outer ply 7b, each extending over substantially the whole width of the tread portion 5. The inner ply 7a is slightly wider than the outer ply 7b, and therefore, the edges 7e of the belt 7 as a whole are those of the widest ply 7a. The belt plies 7a and 7b are made of parallel cords laid at angle of not more than 30 degrees, in this example about 20 degrees with respect to the tyre equator so that the cords in each ply cross those in the other ply. For the belt cords, steel cords are used, but inextensible high modulus strong organic cords such as aromatic polyamide fibre cords or the like can be used. The radially innermost ply 7a directly contacts with the carcass except at its edge portions which are gradually spaced apart from the carcass towards the edge thereof. In the present invention, the carcass line L at an inner pressure of 0 kgf/sq.cm is specifically defined. Here, the carcass line L is defined by the thickness centre line of the carcass 6. The carcass line L comprises, as shown in Fig.2, a crown part Q1 extending between a pair of shoulder points Po, a pair of upper sidewall parts 15 extending radially outwardly from the maximum width points M, and a pair of shoulder parts Q3 extending therebetween. The crown part Q1 is defined by a single radius arc 11 having the centre on the tyre equator C and a first radius of curvature R1. In this embodiment, the crown part Q1 is a convex curve. However, the crown part Q1 can be a concave curve as shown in Fig.4. Also it may be flat. The shoulder points Po are on the carcass line, each axially positioned between one of the axially outer edges 7e of the belt 7 and a position spaced axially inwardly apart from the outer edge 7e by a distance of 20 mm. The axial position of the shoulder point Po roughly corresponds to the position at which the contact relationship between the belt 7 and the carcass 6 changes to a non-contacting relationship. Each of the upper sidewall parts 15 is defined by a single radius arc 12 having a second radius of curvature R2. The centre of this part 15 is located inside the tyre and, in this embodiment, on a straight axial line 21 drawn between the maximum width points M. Each of the shoulder parts Q3 extends from one of the edges of the crown part Q1 or one point Po to the radially outer edge of each sidewall part Q2, and each part Q3 is defined by a single radius arc 13 having a third radius of curvature R3. The centre of this part Q3 is located inside the tyre. The shoulder part Q3 and crown part Q1 inscribe each other. The shoulder parts Q3 are directly and smoothly connected to the upper sidewall parts 15 without forming any angled corner so that the shoulder part Q3 and upper sidewall part 15 inscribe each other. However, between the shoulder part Q3 and the upper sidewall part 15, it is also possible to dispose another curved line such as an ellipse or the like. The third radius of curvature R3 is in the range of from 0.003 to 0.03 times the first radius of curvature R1, and also the third radius R3 is in the range of from 0.05 to 0.5 times the second radius of curvature R2. More preferably, the third radius R3 is in the range of from 0.005 to 0.010 times the first radius R1 and in the range of from 0.06 to 0.20 times the second radius R2. Accordingly, as shown in Fig.2, the carcass line L is bent sharply at the shoulder part Q3 so as to give a square configuration at the zero pressure condition rather than a conventional round configuration. As the inner pressure increases, the carcass line L changes its shape so that the crown part Q1 and sidewall part Q2 inclusive of the upper sidewall part 15 swell towards the outside of the tyre. However, the shoulder part Q3 is depressed contrarily as shown in Fig.3. In Fig.3, the non-inflated state Y1 is shown as a solid line, and the exaggerated inflated state Y2 is shown as a chain line. As a result, the carcass cord tension is decreased in the shoulder part Q3 in comparison with the other parts, and the vibration transmissibility of the carcass 6 is be decreased. If the third radius of curvature R3 is more than 0.03 R1 or more than 0.5 R2, the shoulder parts Q3 are swelled by tyre inflation in the same manner as the remaining parts Q1 and Q2 as shown in Fig.5, and the decreased tension part can not be formed. On the contrary, if the third radius of curvature R3 is less than 0.003 R1 or less than 0.05 R2, it is difficult to manufacture such a tyre. In the lower sidewall part 16 which extends radially inwardly from the maximum width point M to the bead core 2, an outer part 16A extending radially inwardly from the maximum width point M is defined by an extension of the above-mentioned single radius arc 12 thereby having the same radius as the second radius of curvature R2. An inner part 16C extending along the axially inside of the bead apex 8 is curved such that the centre thereof is located on the outside of the tyre. A middle part 16B extending between the convex outer part 16A and convex inner part 16C connects smoothly those parts each other, whereby this middle part serves as an inflection point. In the case of a passenger tyre whose nominal section width is from 95 to 315 mm, the first radius of curvature R1 is in the range of from 600 to 5000 mm. The third radius of curvature R3 is preferably not less than 18 mm, more preferably not less than 20 mm. If the nominal section width is under 200 mm, the R1 is preferably 600 to 3000 mm. If the nominal section width is 200 mm or more, the R1 is preferably 1000 to 5000 mm. Here, the nominal section width is of the standardised metric tyre size designation such as JIS-D4204 in Japan. Fig.6 shows the difference in vibration transmissibility between a conventional tyre and an example tyre of the present invention in which the cord tension is decreased in the shoulder parts Q3 as explained above. The example tyre and conventional tyre had the same construction except the carcass line. The tyre size was 205/65VR15, and the above-mentioned radii R1, R2 and R3 were as follows. Tyre Ex. Con. R1 (mm) 1500 1000 R2 (mm) 65 65 R3 (mm) 24 35 In the test, the tyre inflated to normal pressure was mounted on a test car, and the tread portion was hit on the tyre equator with a hammer giving a 10 kg force. The vibration generated was measured at the wheel axis, and a frequency analysis was made. The results are shown in Fig.6. As shown in this figure, the vibration transmissibility of the tyre of the invention was lessened which reduced road noise. In particular, the transmissibility was effectively decreased around 250 Hz. Through the test, it was confirmed that the road noise could be reduced by decreasing the carcass cord tension at the shoulder parts. However, it was also discovered that the vibration transmissibility could be further decreased by decreasing the rubber thickness in the tyre shoulder part Q3. The rubber thickness is therefore, preferably decreased in the tyre shoulder portions. This reduced thickness part 17 is, as shown in Figs.1 to 2, formed in a region J2 which is between a point P1 on the carcass line L at the same axial position as that of the axially outer edge 7e of the belt 7, and a point P2 on the carcass line L extending 20 mm along the carcass line L from the point P1 towards the sidewall portion 4. In the reduced thickness part 17, the rubber thickness T is set in the range of from 1.5 mm to 3.5 mm, and the width W of this part 17 is in the range of from 3 mm to 20 mm. Here, the rubber thickness T is measured form the outer surface of the tyre to the carcass normally to the carcass. The width W is measured along the carcass. By providing this thinner part 17, the bending rigidity is decreased in the shoulder part, and the vibration transmissibility is further reduced. In this embodiment, the reduced thickness part 17 is formed by gradually changing the rubber thickness from the radially outside and inside of this part 17. However, it is also possible to change the thickness step-wise. Changing the radii R1, R2 and R3 and thickness T, test tyres of size 205/65VR15 were made and tested for the road noise. The test results are given in Table 1. In the tests, the road noise was measured at given frequencies of 160 Hz and 250 Hz and over all the frequency range. The tyres were mounted on all the wheels of a front wheel driven car and then run on a hot-rolled asphalt road at a speed of 60 km/h to measure the noise with a microphone at the driver's right ear position. Tyre Ref. Ex.1 Ex.2 Ex.3 Ex.4 Ex.5 Ex.6 Ex.7 Ex.8 R1 (mm) 1000 1500 1500 3000 3000 3000 3000 5000 -3000 R2 (mm) 65 65 120 120 230 230 230 300 230 R3 (mm) 35 24 24 24 24 20 20 18 24 R3/R1 0.035 0.016 0.016 0.008 0.008 0.007 0.007 0.0036 0.008 R3/R2 0.538 0.369 0.200 0.200 0.104 0.087 0.087 0.060 0.104 T (mm) 4.5 3.0 3.0 3.0 3.0 3.0 4.5 2.8 3.0 Road noise dB(A) Overall 70.8 69.7 69.4 69.0 68.8 68.4 68.7 68.3 68.6 160 Hz 64.1 63.6 63.5 63.2 63.1 62.8 63.0 62.7 62.8 250 Hz 64.7 63.6 63.2 62.9 62.5 62.0 62.3 61.9 62.3 Through the tests, it was confirmed that the example tyres was remarkably reduced in the 250 Hz noise in comparison with the reference tyre. In comparison with the Ex.1 tyre, the Ex.2 tyre was improved in over all noise by increasing the second radius R2. In comparison with the Ex.2 tyre, the Ex.3 tyre was improved in over all noise by increasing the second radius R1. In comparison with the Ex.3 tyre, the Ex.4 tyre was improved in over all noise and 250Hz noise by increasing the second radius R2. In comparison with the Ex.4 tyre, the Ex.5 tyre was improved in over all noise by decreasing the third radius R3. In comparison with the Ex.5 tyre, the Ex.6 tyre was increased in respect of the rubber thickness T. As a result, the noise level was increased, but better than the reference tyre. In the Ex.7 tyre, the third radius R3 is further decreased, but an effective noise reduction could not be obtained. The Ex.8 tyre was formed such that the curve of the tread crown part was concave in contrast with the former Ex. tyres. By the concave carcass line, the cord tension and noise level were further decreased when compared with the Ex.4 tyre which was different only in the type of the crown curve. As explained above, in the pneumatic tyres of the present invention, the vibration transmissibility of the shoulder part is decreased, and the road noise, especially over 200 Hz range can be effectively reduced.

Claims: A pneumatic tyre comprising a tread portion (5), a pair of axially spaced bead portions (3) with a bead core (2) therein, a pair of sidewall portions (4), a carcass (6) extending between the bead portions (3), a belt (7) disposed radially outside the carcass (6) and inside the tread portion (5), characterised in that in a meridian section of the tyre when mounted on a standard rim but not inflated, the carcass line (L) defined by the thickness centre line of the carcass (6) comprise a tread crown part (Q1) beneath the belt (7), a pair of upper sidewall parts (15) radially outward of the maximum carcass width position, and a pair of shoulder parts (Q3) between the tread crown part (Q1) and the upper sidewall parts (15), the tread crown part (Q1) having a first single radius of curvature R1, the upper sidewall parts (15) having a second single radius of curvature R2, and the shoulder parts (Q3) having a third single radius of curvature R3, the third radius of curvature R3 being in the range of from 0.003 to 0.03 times the first radius of curvature R1, the third radius of curvature R3 being in the range of from 0.05 to 0.5 times the second radius of curvature R2, the axial position of the boundary (Po) between the tread crown part (Q1) and each shoulder part (Q3) being between the axial position of each edge (7e) of the belt (7) and a position which is 20 mm axially inward thereof. A pneumatic tyre according to claim 1, characterised in that the tread crown part (Q1) is a convex arc. A pneumatic tyre according to claim 1, characterised in that the tread crown part (Q1) is a concave arc. A pneumatic tyre according to claim 1, 2 or 3, characterised in that a reduced rubber thickness (17) part is provided in each shoulder region which is defined between a first point (P1) on the carcass line at the same axial position as each of the belt edges (7e) and a second point (P2) on the carcass line spaced apart radially inwardly from the first point by 20 mm along the carcass line (L), the reduced rubber thickness part (17) having a rubber thickness of not more than 3.5 mm and a width of not less than 3 mm, said rubber thickness being measured normally to the carcass line (L) from the tyre outer face to the carcass (6), and said width being measured along the carcass (6).

Inventors: TANAKA MASATOSHI, TANAKA, MASATOSHI
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 1994
Main classification: B60C11
IPC codes: B60C3